I decided to investigate our Tier 10 4-piece bonus, considering that I was already halfway there with my two current pieces. However, I was also skeptical about how valuable the 4-piece bonus could be, considering that cast time reductions and haste interact in interesting ways. Well, there's that, and the fact that our tier set was itemized by someone who clearly enjoyed watching us squirm. With a complete lack of a haste/MP5 piece and two crit/MP5 pieces, our 4-piece bonus had better be really amazing to convince me to drop my off set pieces. Read on to find out what I decided.

I'd suggest picking up the helm and shoulder slots of our set first. The helm is great since it's the only piece of the set that isn't missing a socket, and haste/crit is pretty good. I recommend the shoulders because they're cheap on Emblems, and the only plate shoulders above ilvl 251 with haste on them anyway. As a holy paladin, this is where you should be spending your first 155 Emblems of Frost, because the mana return is so tangible that it can literally make or break any encounter where you're running out of mana. We're not talking about a 50 MP5 difference: we're talking about 25% mana in your bar vs running out of mana and the tank dying.

4-piece bonus

Okay, so we've confirmed Lightsworn is decent enough to pick up two pieces of it, but what about shooting for the 4-piece bonus? It seems great, like a throwback to the old Infusion of Light that reduce our Holy Light cast time on a Holy Shock crit, except without the requirement of a crit. So why was I skeptical about the value of the bonus? The reason is simply is that our gear has improved by miles since then, and the value of a cast time reduction effect is weakened when copious amounts of haste are applied.

If you have Light's Grace, you've seen this effect before. LG is supposed to reduce the cast time of Holy Light by 0.5 seconds every cast, and it does that job perfectly. Holy Light starts with a 2.5 second cast time, and is dropped to an even 2.0 seconds with Light's Grace active. Let's add haste to the equation now: Light's Grace still reduces the cast time by 0.5 seconds, and sets the cast time to 2.0 seconds. Two seconds then becomes the 'base cast time' as far as haste is concerned, so we see less of a cast time reduction benefit from haste than we're used to seeing.

The faster your spell is, the fewer seconds that haste shaves off of the cast time of that spell. Haste still scales linearly: more haste is always good, and one point of haste is just as good as the preceding point of haste, at least as far as pure Holy Light throughput is concerned. However, our throughput is already so great that we're not worried about our HPS any more, we're concerned with our 'time until healed' or TUH. We need to reduce our TUH to maximize our ability to raid heal, which is really our only true weakness.

My verdict

Jackslammer of Scilla did a great write-up of the actual cast time reductions we can expect from the 4-piece Tier 10 bonus, and frankly, the results aren't good. Assuming we're at the haste soft cap of 676 (which means a 1 second GCD) from our non-set slots and have all the standard raid buffs, our Holy Lights are actually slower when using 4-piece Tier 10, due to losing haste by equipping the set. This means our TUH value increases (which is bad) when just casting Holy Light normally. The fact that our baseline throughput and reactivity (yes, I'm making up this word) take a hit by using Lightsworn makes the set very hard to swallow.

Now, we do see a cast time reduction on Holy Light when it immediately follows a Holy Shock, to the tune of 0.138 seconds in the example above. That's less than half of what the set bonus advertises, and means it only makes us slightly more reactive than normal. Consider the period where Holy Shock is on cooldown: you're casting slower Holy Lights than usual, and you have to Holy Shock in order to make up the difference. If you're not Shocking on cooldown, you're actually casting slower than you would be otherwise.